The use of DNA in forensics is one of the most powerful tools available to law enforcement officials, prosecutors, and defendants. One use of DNA evidence has been to prove conclusively that DNA samples obtained from a crime scene did not originate from a suspect or from a person wrongly convicted of the crime. For example in several highly publicized cases, persons on death row have been exonerated from their crimes based primarily on evidence provided by DNA analysis of biological samples found at the crime scene. In fact, the state of Illinois has recently issued a moratorium on capital punishment in the state as a result of convicted persons on death row freed on the basis of DNA evidence. In these cases, DNA evidence can determine with absolute certainty that two DNA samples originated from two different individuals and not from the same individual.
Conversely, to determine that two DNA samples originated from the same individual, DNA analysis can provide an extremely high probability of a match. However, it can not be determined with absolute certainty that two DNA samples originated from the same individual.
Other examples of the use of DNA in forensics include identifying victims of crime and accidental catastrophes. In many cases, DNA at crime scenes or from natural or accidental catastrophes is degraded, thus rendering analyses of the DNA extremely difficult. One of the many techniques used in the analysis of DNA in forensics is restriction fragment length polymorphism (RFLP) analysis. Briefly, the analysis involves the cleavage of a sample of DNA by a restriction enzyme and the subsequent analysis of the size of the resulting cleavage fragments by polyacrylamide gel electrophoresis (PAGE). Since individuals contain regions of their genome that vary in size from person to person (variable number of tandem repeats; VNTR), a pattern of cleavage products on a gel representing the different sizes of cleavage products provides a DNA profile for each person.
When the pattern of cleavage products resulting from RFLP analysis differs for two different DNA samples, it is conclusive that the DNA samples originated from different people. However, when the pattern is identical for two DNA samples, a statistical analysis is necessary to determine the probability that the two samples are from the same person.
Unfortunately, a problem that arises with forensics analyses of DNA samples is the degradation and/or fragmentation of the DNA resulting from exposure to the environment and microorganisms which contain DNA endonucleases. The problem of degraded DNA is also faced by researchers who are analyzing the DNA obtained from frozen, extinct or extremely rare organisms. Therefore, there is a need for methods that reconstitute fragmented degraded DNA into DNA useful for genetic analysis and manipulation.